Broadband MXN optical fiber couplers and method of making

ABSTRACT

A broadband optical fiber coupler comprising at least three continuous optical fibers fused at a region of coupling, at least one of the continuous optical fibers being dissimilar to others of the continuous optical fibers in the region of coupling, the region of coupling being of a length and the dissimilarity being of a degree to provide broadband response over a predetermined range of wavelengths. A related method for making a broadband optical fiber coupler is also disclosed.

FIELD OF THE INVENTION

The invention relates generally to passive optical couplers, and, moreparticularly, to biconically tapered optical couplers with more than twooutputs operable over a broad wavelength region.

BACKGROUND OF THE INVENTION

Fiber optics are widely used in many diverse applications, includingtelecommunication systems, instrumentation and sensing operations. Anexample of such an application is a multi-access opticaltelecommunications network. In such a network, optical fiber connects anumber of users or subscribers to a central office using passivecouplers. This type of network is particularly attractive since thereare typically no active optical devices located outside of the centraloffice or subscriber locations.

An optical fiber typically includes an inner glass or plastic coresurrounded by an outer cladding similarly of glass or plastic. The innercore has a relatively higher index of refraction than the cladding thusallowing light to be transmitted through the core very efficiently.Light may be transferred or split between separate fibers through theuse of an optical fiber coupler. One extensively used type of opticalfiber coupler is a fused biconically tapered (FBT) coupler. In onemethod of producing such a fiber optic coupler, a number of opticalfibers are held in axial alignment and elongated while being heated.This process creates a biconically tapered region or waist wherein theoptical fibers are fused together and the optical signals from one ormore optical fibers can be coupled to or split between other opticalfibers.

The basic optical performance of optical fiber couplers can be describedby three fundamental quantities: excess loss, splitting loss anduniformity. The excess loss, expressed in decibels (dB), is a measure ofhow much light or optical energy is lost in the coupling process. Excessloss is defined as the ratio of the total output power to the amount ofoptical power launched into the input of the coupler. The ratio of theoptical power in one of the output fibers relative to the total opticalpower output over all the output fibers is known as the splitting loss.The splitting loss is also often expressed in decibels. Another termoften used to characterize the actual optical performance of couplers is"uniformity". Uniformity is a measure of the spread in the splittingratios from the ideal values. It is also expressed in decibels and isdefined as the difference between the maximum and minimum values ofsplitting loss.

Many of the optical fiber couplers in use today are designed to operateeffectively over only a narrow range or "window" of wavelengths. Themost common wavelengths of interest for telecommunication applicationsare those centered around 1300 nm or 1550 nm. These optical fibercouplers, often called single window couplers, essentially provide equalsplitting of light from one or more input fibers to a number of outputfibers at a preselected wavelength.

However, the splitting loss of each output port for such a single windowcoupler changes as a function of the wavelength of the transmittedlight. In particular, as the wavelength of the transmitted light variesfrom the center of the wavelength window, the optical power in theoutput fibers (i.e., splitting loss) tend to diverge from the idealvalue and the uniformity becomes quite large. This behavior typicallylimits the use of such single window couplers to within ±20 nm of thecenter of the wavelength window.

In many optical fiber telecommunication applications, operation in twowavelength windows, such as 1300 nm and 1550 nm, is required in order toprovide both telephony and broadband services. In these applicationsbroadband optical fiber couplers, which exhibit a relatively constantsplitting loss over a broad range of wavelengths, are required.

One technique of fabricating a broadband optical fiber coupler requiresintroducing a dissimilarity in one of two optical fibers in the couplingregion. This technique, however, has been limited to producing couplerswith only two output fibers. Other techniques which allow splitting overmore than two output fibers are known, but they are limited to 1×Ncouplers. One such technique involves inserting identical strippedoptical fibers into a tight fitting outer sheath consisting of a glasscapillary tube. The entire structure is then heated and stretched toachieve coupling. By varying the number of fibers inserted into theglass sheath and their relative positions and separations, certainbroadband 1×N couplers can be fabricated. However, most of these 1×Ncouplers require either the use of N+1 fibers or the use of dummy fibersthat have no light guiding cores and as such are not conventionaloptical fibers.

It would be desirable to provide an M×N broadband optical fiber coupler,where M ranges from 1 to N and N is greater than 2, that is easilyfabricated and has good uniformity and splitting ratios and low excesslosses.

SUMMARY OF THE INVENTION

The broadband M×N optical fiber couplers of the present inventionutilize a relatively easy fabrication technique which involves combiningdissimilar optical fibers. The desired dissimilarity in the fibers maybe achieved by a number of methods prior to assembly. Such methodsinclude reducing the size of the optical fibers by etching, pretaperingor polishing techniques, doping the optical fiber core or claddingdifferently, diffusing existing dopants into other areas of the fiber,depositing additional glass material on the optical fiber and exposingthe fibers to electromagnetic or nuclear radiation. Alternatively,fibers designed to have different parameters, such as core and claddingdiameters, propagation constants and refractive index profiles, may beused.

In accordance with one aspect of the present invention, a process forfabricating a broadband fiber optic coupler includes creating adissimilarity in at least one of at least three continuous opticalfibers, arranging the continuous optical fibers in relative axialalignment and in intimate contact, and heating, elongating and fusingthe continuous optical fibers together to achieve coupling.

In accordance with another aspect of the invention, a broadband opticalfiber coupler includes at least three continuous optical fibers fused ata region of coupling, at least one of the continuous optical fibersbeing dissimilar to others of the continuous optical fibers in theregion of coupling, the region of coupling being of a length and thedissimilarity being of a degree to provide broadband response over apredetermined range of wavelengths.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and the annexed drawings setting forth in detail a certainillustrative embodiment of the invention, this being indicative,however, of but one of the various ways in which the principles of theinvention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic illustration of a 1×3 broadband fiber opticcoupler and associated optical fibers constructed in accordance with theinvention;

FIG. 2 is a cross section of the coupler viewed along the line 2--2 inFIG. 1;

FIG. 3 is a graph of normalized coupled power as a function of couplinglength for the optical fibers coupled by the coupler of FIG. 1 for theoptical wavelengths of 1300 nm and 1550 nm;

FIG. 4 is a schematic illustration of a 2×4 broadband fiber opticcoupler and associated optical fibers constructed in accordance with theinvention;

FIG. 5 is a cross section of the coupler viewed along the line 5--5 inFIG. 4;

FIG. 6 is a graph of normalized coupled power as a function of couplinglength for the optical fibers coupled by the coupler of FIG. 4 for theoptical wavelengths of 1300 nm and 1550 nm;

FIG. 7 is a graphical illustration of splitting loss as a function ofwavelength for one input of a fused broadband 2×4 optical fiber couplerconstructed in accordance with the present invention;

FIG. 8 is a graphical illustration of splitting loss as a function ofwavelength for one input of a standard single window 4×4 fused opticalfiber coupler intended for use in the 1550 nm wavelength region;

FIG. 9 is a graphical illustration of splitting loss as a function ofwavelength showing the wavelength response of a fused broadband 1×3fiber optic coupler constructed in accordance with the presentinvention; and

FIG. 10 is a graphical illustration of splitting loss as a function ofwavelength for one input of a standard single window 3×3 fused opticalfiber coupler intended for use in the 1550 nm wavelength region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

With reference to the figures, and initially to FIG. 1, there isillustrated a broadband 1×3 optical fiber coupler 10 in accordance withthe present invention. The coupler 10 optically couples light from aninput optical fiber 1 to output optical fibers 1, 2 and 3. Note that theinput optical fibers 1, 2 and 3 are continuous throughout the couplerand become the output fibers downstream of the coupling region 10. Inthis embodiment only fiber 1 is used as the input fiber with fibers 2and 3 unused on the input side 11 of the coupler. The optical fibers 1,2 and 3 are each made of a light transmitting glass core surrounded by asimilar glass cladding with a relatively lower index of refraction thanthe core. An example of a suitable optical fiber is standard single modetelecommunications fiber, such as that made by Corning Glass Works andidentified as SMF28™. Alternatively, a multimode optical fiber could beused. The optical fibers are fused together in the coupling region,preferably using a fused biconical taper technique as will be describedmore fully below.

Referring to FIG. 2 there is shown a cross-section of the optical fibers1, 2 and 3 in the fused area of the coupler 10. The cores of opticalfibers 1, 2, 3 are denoted by reference numerals 1', 2' and 3',respectively. The claddings (referenced generally as 12) of the opticalfibers 1, 2 and 3 are coalesced by the fusion process and occupy thearea between the fiber cores 1', 2' and 3' and areas immediatelysurrounding the cores (the boundary of the cladding 12 is not shown).For the purpose of discussion the desired dissimilarities in the opticalfibers are assumed to be achieved by using fibers having different corediameters.

The core 1' of input fiber 1 is slightly smaller in diameter than thecores 2' and 3' of optical fibers 2 and 3, respectively. Cores 2' and 3'have equal diameters in this example. This dissimilarity in the corediameters provides the desired dissimilarity in the propagationconstants of the optical fibers when coupled. In reality thedissimilarity in the fibers can be achieved through a variety ofmethods, a dissimilarity in the core diameters of the optical fibersbeing only one method. Moreover, while the core 1' is shown as having arelatively smaller diameter than cores 2' and 3', other combinations ofdiameter dissimilarities can be employed.

The exchange or coupling of optical power among the fibers in couplersmay be described using coupled mode theory. Coupled mode theory is basedon a perturbation approach which assumes that the optical fibers areelectromagnetically well isolated. A more in-depth discussion of coupledmode theory can be found in "Optical Waveguide Theory" by A. W. Snyderand J. D. Love, Chapman and Hall (1983), Chapter 29.

The coupling behavior of the 1×3 optical fiber coupler 10 is describedby the following set of two coupled mode equations: ##EQU1## where β₁ isthe propagation constant of the fundamental mode of input fiber 1 and β₂is that of identical fibers 2 and 3; C_(a) is the coupling constantbetween the optical fibers; a₁ is the modal amplitude of fiber 1 and a₂is the modal amplitude of fibers 2 and 3; and z is the interactionlength over which the coupling occurs.

Substituting the expression a_(n) =A_(n) exp(-jλz) into the set ofequations 1, leads to the characteristic eigenvalue equation in matrixform: ##EQU2## The matrix above is solved numerically to determine itseigenvalues. The associated eigenvectors are subsequently determinedalgebraically. The modal amplitudes a₁ and a₂ can then be represented bythe following two equations:

    a.sub.1 =A.sub.11 exp(-jλ.sub.1 z)+A.sub.12 exp(-jλ.sub.2 z)

    a.sub.2 =A.sub.21 exp(-jλ.sub.1 z)+A.sub.22 exp(-jλ.sub.2 z)(3)

where λ₁ is one eigenvalue and A_(n1) is its associated eigenvectorafter applying the boundary condition for the normalized coupled powerfor the fibers at a coupling length z=0. Representing the normalizedcoupled power as Pn, where "n" is the optical fiber, the boundarycondition at the point z=0 is P1=1 and P2=P3=0.

The coupled power carried by each of the three fibers plotted as afunction of the coupling length z, for both 1300 and 1550 nm, isgraphically depicted in FIG. 3. At a coupling length z equal toapproximately 3.7 mm, the coupler 10 exhibits broadband behavior byproviding an equal three-way split for both wavelengths simultaneously.

While the above discussion is for a broadband 1×3 optical fiber coupler,the example is illustrative and the principles can be extended to other1×N arrays where N is greater than 2. Further, the invention can beapplied to both single mode and multimode optical fibers

Referring now to FIG. 4, there is shown a broadband 2×4 optical fibercoupler 20 optically coupling fibers 4, 5, 6 and 7. Note that all fourfibers 4, 5, 6 and 7 are continuous throughout the coupler 20 thusproviding four output fibers on the right or output side 21 of thecoupler 20. In this embodiment, optical fibers 4 and 6 to the left sideor input side 22 of the coupler 20 in the figure represent the inputfibers with fibers 5 and 7 being unused on the input side.

Again, the optical fibers 4, 5, 6 and 7 are each made of a lighttransmitting glass core surrounded by a similar glass cladding with arelatively lower index of refraction than the core, for example SMF28™by Corning Glass Works. The optical fibers are also fused together inthe coupling region, preferably using the fused biconical tapertechnique described more fully below.

FIG. 5 illustrates a cross-section of the optical fibers 4, 5, 6 and 7in the area of the coupler 20. The cores of optical fibers 4, 5, 6 and 7are denoted by like primed reference numerals. The claddings (referencedgenerally as 23) of the optical fibers 4, 5, 6 and 7 are merged by thefusion process and occupy the area between the fiber cores 4', 5', 6'and 7' and areas immediately surrounding the cores (the boundary of thecladding 23 is not shown).

As illustrated in FIG. 5, the core 4' of input fiber 4 is slightlylarger in diameter than the cores 5' and 7' of optical fibers 5 and 7,respectively, with the diameter of core 6' of optical fiber 6 beingsmaller than the diameter of cores 5' and 7'. The cores 5' and 7' are ofequal diameter in this example. Once again, for the purpose ofdiscussion, the dissimilarities in the optical fibers are beingillustrated by different core diameters, although the desireddissimilarities in the propagation constants of the optical fibers oncecoupled could be accomplished in many other ways, some of which areenumerated herein. Other combinations of core diameter dissimilaritiesalso can be employed which meet the core diameter ratio requirements setforth in equation 4 below.

The broadband 2×4 fiber coupler 20 is implemented with core diameters4', 5', 6' and 7' determined according to the following ratio: ##EQU3##where ρ₄, ρ₅, ρ₆, and ρ₇ are the diameters of the cores 4', 5', 6', and7', respectively.

The behavior of coupler 20 can be described by the following set ofthree coupled mode equations: ##EQU4## where β₄ and β₆ are thepropagation constants of fibers 4 and 6, respectively, and β₅ is thepropagation constants of fibers 5 and 7 (assumed to be identical); C_(a)is the coupling constant between the optical fibers; a₄ and a₆ are themodal amplitudes of fibers 4 and 6, respectively, and a₅ is the modalamplitude of fibers 5 and 7; and z is the interaction length over whichthe coupling occurs.

This leads to the characteristic equation in matrix form: ##EQU5## whichis solved in the manner described in the preceding section.

The optical power carried by each of the four fibers 4, 5, 6 and 7 isplotted in FIG. 6 as a function of the coupling length z for both 1300and 1550 nm light. The figure shows that at a coupling length z equal to5.5 mm, the coupler 20 provides an equal four-way split at 1300 and 1550nm simultaneously, thus enabling the broadband mode of operation of thistype of coupler.

The broadband fiber optic couplers of the present invention arefabricated in a manner very similar to that used to construct standard3×3 and 4×4 single window fused optical fiber couplers, with theaddition of a step to introduce a dissimilarity in one or more of thefibers. The 1×3 and 2×4 optical fiber couplers described herein werefabricated as described below using two different methods to achieve thedissimilarities in the fibers. Other configurations of fused broadbandcouplers in accordance with the present invention could be fabricated inmuch the same way.

For the broadband 2×4 optical fiber coupler, four equal lengths ofstandard single mode telecommunications fiber (Corning SMF28™) were cut.Twenty-five mm long sections of the protective buffers of each fiberwere stripped, and the exposed glass portions cleaned. To create thedesired dissimilarities in the fibers for the 2×4 optical fiber coupler,the appropriate fiber diameters were reduced with a glass etchant.Fibers 5, 6 and 7 were etched appropriately to maintain the relativeoutside fiber diameter ratios given in equation 4. Fiber 4 was notetched. For the broadband 1×3 optical fiber coupler, the dissimilaritiesin the optical fibers were achieved by pretapering the fibers. However,as discussed earlier, the desired dissimilarities in the fibers foreither of these couplers could also have been achieved by a number ofother methods. For instance, doping the optical fiber core or claddingdifferently, diffusing existing dopants into other areas of the fiber,using fibers of different diameters, using fibers made with differentoptical propagation constants, or using fibers made with differentrefractive index profiles. If the desired dissimilarities in the fibersare to be achieved by reducing the outside diameter, convenient methodsare etching, pretapering, and polishing. Dissimilarities in the fibercan also be achieved by the deposition of additional glass or plasticmaterial on the optical fiber.

The fibers were then mounted parallel to each other in a slidingmechanical fixture. The fibers are then braided and brought together ina closed bundle formation in order to achieve relative axial alignmentand intimate side-by-side contact. Other mechanical means, fixtures,and/or adhesives could have also been used to achieve this alignment andcontact.

The fiber bundles were fused using a fused biconical tapering processwherein the fibers were heated and elongated to form the coupler. Duringthe fusion process, light was launched into an input fiber, and theoptical outputs of the output fibers were monitored using germaniumphotodetectors. The fusion process was terminated when the input lightwas split evenly among the output fibers. The fusion process also can bemonitored by using a 2×1 optomechanical switch alternatively to launchlight at 1300 nm and 1550 nm into the input fiber during the fusionprocess. In this case the fusion process would be terminated once aminimum spread in the splitting loss is detected at both wavelengths.The heat for fusion can be supplied by any heat source with sufficientenergy to achieve the temperature required for fusing, such as a gaseousflame, an electric furnace, or optical energy, i.e., a CO₂ laser.

In the preferred embodiment the coupler was secured into a silicasubstrate using an adhesive. Materials other than silica may be used forthe substrate. The coupler was then inserted and sealed in a protectiveINVAR tube typically three millimeters in diameter and sixty-fivemillimeters in length. An outer housing other than the INVAR tube mayalso be used. Similarly, other attachment methods may be employed, suchas laser welding or glass sealing materials, to secure the coupler tothe silica substrate.

The performance of the fused broadband 2×4 and 1×3 optical fibercouplers was tested using a white light source and an optical spectrumanalyzer. FIG. 7 is a graphical illustration of splitting loss as afunction of wavelength for a broadband 2×4 optical fiber couplerfabricated in accordance with the present invention. The splitting lossfor each of the four output fibers is relatively constant over the broadwavelength spectrum from 1200 nm to 1600 nm, and the uniformity is about1.4 dB over the entire wavelength region. For comparison, thecharacteristics of a standard single window 4×4 fused coupler intendedfor use in the 1550 nm wavelength region is depicted in FIG. 8. Thesplitting losses of the standard 4×4 coupler diverge substantially fromthe ideal value of approximately 6 dB as the wavelength of input lightvaries from the center (1550 nm) of the wavelength window. Conversely,the splitting loss and uniformity of the 2×4 broadband coupler of thepresent invention exhibit minimal sensitivity to the operatingwavelength as seen in FIG. 7.

FIG. 9 is a graphical representation of splitting loss as a function ofwavelength for a fused broadband 1×3 optical fiber coupler in accordancewith the present invention. The uniformity of the splitting losses overthe three optical fibers is approximately 0.5 dB at 1300 nm andapproximately 0.1 dB at 1550 nm. Furthermore, the maximum uniformitythroughout the wavelength region of 1200 nm to 1600 nm is less than 1dB. In contrast, the corresponding response of a standard single window3×3 fused fiber coupler intended to be used in the 1550 nm region isillustrated in FIG. 10. The splitting losses of the standard 3×3 couplerdiverge substantially from the ideal value of approximately 4.8 dB asthe wavelength of input light varies from the center (1550 nm) of thewavelength window. The substantially reduced wavelength sensitivityprovided by the 1×3 broadband coupler of the present invention over astandard 3×3 coupler is clear from a comparison of FIGS. 9 and 10.

A comparison of the wavelength responses for the dual window opticalfiber couplers constructed in accordance with the invention with thewavelength responses of standard fused optical fiber couplers indicatesthat the couplers of the present invention operate more uniformly over abroader wavelength spectrum than do the tested conventional devices. Theoptical couplers of the present invention also offer advantages overother known broadband optical fiber couplers in that standardfabrication equipment can be used and the complexity associated withusing a sheathing capillary tube is avoided. Moreover, additionallengths of fiber do not need to be spliced or pigtailed in order toaccess the coupler, such as is the case with planar waveguide splitters,and the disadvantages associated with having to concatenate the requirednumber of standard 2×2 broadband couplers also is avoided.

We claim:
 1. A process for fabricating a 2×4 broadband fiber opticcoupler, comprising the steps of:a) arranging four optical fibersrelatively axially in intimate contact, with two of the four fibersbeing both launch fibers and output fibers, at least two of the fouroptical fibers having unequal core diameters in the area of coupling andthe four optical fibers having core diameters in the area of couplingsatisfying the relationship: ##EQU6## where ρ_(n), with n being aninteger between 1 and 4, represents the diameter of the core of aparticular optical fiber; and b) heating, elongating and fusing saidfour optical fibers together to achieve broadband coupling.
 2. A 1×3broadband optical fiber coupler comprising three optical fibers fused ata region of coupling, with one of the three fibers being a launchoptical fiber which is continuous through the region of coupling toprovide an output fiber, the launch optical fiber being dissimilar tothe other two optical fibers in the region of coupling, said region ofcoupling being of a length and said dissimilarity of said launch opticalfiber being of a degree to provide broadband response over apredetermined range of wavelengths.
 3. The process of claim 1, furtherincluding launching light into at least one of said optical fibers,monitoring the light output from at least one of said optical fibers,and terminating said step of heating, elongating and fusing when saidoutput reaches a specified value.
 4. The process of claim 1, whereinsaid step of heating, elongating and fusing includes using a biconicaltapering technique.
 5. The process of claim 1, wherein said opticalfibers are single mode optical fibers.
 6. The process of claim 1,wherein said optical fibers are multimode optical fibers.
 7. The processof claim 1, wherein said step of arranging includes arranging said fouroptical fibers generally in a diamond configuration.
 8. The coupler ofclaim 2, wherein said launch optical fiber has a different diameter thansaid other two optical fibers at the region of coupling.
 9. The couplerof claim 2, wherein said launch optical fiber has a differentpropagation constant than said other two optical fibers at the region ofcoupling.